Light olefins such as ethylene, propylene and so on are substances that are important as raw materials of various chemicals. Conventionally, decomposition by heating among an external heat-type tubular furnace and under the atmosphere of steam with the use of materials of gaseous hydrocarbon such as ethane, propane, butane and so on or of liquid hydrocarbon such as naphtha and so on is adopted broadly as a process for producing light olefin. However, the decomposition by heating has economic disadvantages such that it requires an elevated temperature of 800° C. or higher in order to raise olefin yield, and accordingly, that it must employ expensive materials for the apparatus.
Therefore, a catalytic cracking method of hydrocarbons with the use of catalyst has been discussed in various ways. Among these, many examples are reported about the cases of employing solid acids, particularly penta-sil type zeolite such as ZSM-5, because it is advantageous in achieving relatively high ethylene yield and propylene yield (about 10 to 30% by mass respectively) at a reaction temperature of around 500 to 700° C.
For example, catalytic cracking methods employing ZSM-5 type catalyst having specific acidity and acid strength (refer to, for example, Japanese Unexamined Patent Application Laid-Open Nos. Hei 3-504737 and Hei 6-346062), and employing ZSM-5 type catalyst containing transition metals such as copper, cobalt and so on (refer to, for example, Japanese Unexamined Patent Application Laid-Open Nos. Hei 2-1413 and Hei 2-184638) are disclosed. Further, catalytic cracking methods employing ZSM-5 type catalyst containing rare earth element (refer to, for example, U.S. Pat. Nos. 5,232,675 and 5,380,690, European Patent No. 727404, Japanese Unexamined Patent Application Laid-Open Nos. Hei 11-180902 and Hei 11-253807) are disclosed. Problems are generally reported that carbon (coke) generated by excessive decomposition of hydrocarbons or hydrogen migration reaction adheres on the catalyst in reaction employing these zeolite catalyst, and causes loss of catalyst activity. Accordingly, continuous recovery by fluidized bed system reaction (refer to, for example, U.S. Pat. Nos. 5,232,675 and 5,380,690 and European Patent No. 727404) or reaction under the coexistence of steam in large amount (refer to, for example, Japanese Unexamined Patent Application Laid-Open Nos. Hei 11-180902 and Hei 11-253807) becomes necessary. However, there are problems that dealumination from zeolite lattice is caused by high-temperature steam being generated with combustion of steam or carbon introduced for the purpose of recovery, thereby induces permanent loss of catalyst activity. Therefore, improvement in hydrothermal stability of zeolite is unavoidable for using these zeolite catalysts industrially for a long time.
Hydrothermal stability of zeolite generally improves depending on an improvement of its crystallinity or on an augmentation of SiO2/Al2O3 ratio. For example, a high silica type zeolite with SiO2/Al2O3 mole ratio of 10 or more is reported as superior in heat resistance (refer to “Studies in surface Science and Catalysis”, 1996, volume 105, p. 1549). However, these catalysts lack sufficient durability in application for a long period under industrial conditions, and therefore, various kinds of improvement are researched.
For example, ZSM-5 type zeolite of high hydrothermal stability containing transition metals such as Fe, Cu, Co, Ni, Cr, Mn, etc. and potassium or cesium (refer to, for example, Japanese Unexamined Patent Application Laid-Open Nos. Hei 4-50115, Hei 4-55310, Hei 4-78443 and Hei 4-78444) is not employable as a cracking catalyst because it contains alkali metal such as potassium, cesium, etc., and because acid strength is poor. Although there is a paper describing that ZSM-5 type catalyst containing Mn improves hydrothermal stability, catalytic property in catalytic cracking is indistinct (refer to “Studies in surface Science and Catalysis”, 1996, volume 105, p. 1549). Further, although Japanese Unexamined Patent Application Laid-Open No. Hei 8-299166 discloses catalytic cracking reaction of hydrocarbon with the use of ZSM-5 type catalyst containing Mn and/or Re, there is no description about durability, and olefin selectivity is so small as 40% or lower because there are much aromatic by-product.
Furthermore, although fluid catalytic cracking methods with the use of catalyst consisting of zeolite and Mn dispersed in inorganic matrix are disclosed, they are aiming production of gasoline and reporting that containing Mn among zeolite is not preferable because octane value of gasoline decreases (refer to Japanese Unexamined Patent Application Laid-Open Nos. Hei 8-299166 and Hei 11-300210). Moreover, although U.S. Pat. No. 4,956,075 discloses that gasoline with high octane value is obtained by catalytic cracking method with the use of large pore size (Y type) zeolite modified with Mn and rare earth element, not only ethylene and propylene are almost not obtained but also there is no description about durability.
Improvement in hydrothermal stability of zeolite by other modifier such as, for example, Zr or so is disclosed, however, it is discussed about under relatively low temperature of around 400° C., and it is not clear about combined effect with rare earth element or about whether it can be applied effectively in producing olefin or not (refer to, for example, Japanese Unexamined Patent Application Laid-Open No. Hei 3-505844).
As the foregoing description, any catalytic cracking method in which catalytic cracking of hydrocarbon contributes for producing olefin with high yield and stably for a long term is not established yet.